The invention relates to the use of malic acid diesters to reduce the surface tension in water-based systems.
The ability to reduce the surface tension of water is of great importance in waterborne coatings, inks, adhesives, fountain solutions and agricultural formulations because decreased surface tension translates to enhanced substrate wetting in actual formulations. Surface tension reduction in water-based systems is generally achieved through the addition of surfactants. Performance attributes resulting from the addition of surfactants include enhanced surface coverage, fewer defects, and more uniform distribution. Equilibrium surface tension performance is important when the system is at rest. However, the ability to reduce surface tension under dynamic conditions is of great importance in applications where high surface creation rates are utilized. Such applications include spraying, rolling and brushing of coatings or spraying of agricultural formulations, or high speed gravure or ink-jet printing. Dynamic surface tension is a fundamental quantity which provides a measure of the ability of a surfactant to reduce surface tension and provide wetting under such high speed application conditions.
Traditional nonionic surfactants such as alkylphenol or alcohol ethoxylates, and ethylene oxide (EO)/propylene oxide (PO) copolymers have excellent equilibrium surface tension performance but are generally characterized as having poor dynamic surface tension reduction. In contrast, certain anionic surfactants such as sodium dialkyl sulfosuccinates can provide good dynamic results, but these are very foamy and impart water sensitivity to the finished coating.
In addition to the development of high-performance surfactants, there is considerable interest in the industry in surfactants with improved environmental characteristics. Environmental concerns have led to an increased use of environmentally compatible surfactants as alternatives have become available. In addition, the use of less favorable products, such as alkylphenol ethoxylate (APE) surfactants, has declined. This is, in part, due to the poor environmental characteristics of APE surfactants, such as incomplete biodegradation and a suspicion that they may function as endocrine mimics. The demand for high-performance, eco-friendly surfactants has stimulated efforts in new surfactant development. From this work a new family of surfactants, referred to as alkyl polyglycoside (APG) surfactants, has emerged as a readily biodegradable, environmentally-friendly alternative to conventional surfactants. These materials, however, can be foamy and thus, are not suitable for a variety of coating, ink, adhesive and agricultural applications where the generation of foam is undesirable. Moreover, many APG surfactants possess poor color characteristics and are solids or pastes. This latter property complicates handling and necessitates the formation of blends which contain significantly less than 100% active ingredient. Thus, not only is it desirable to obtain surfactants which exhibit excellent surface tension reducing capabilities and low foam under dynamic application conditions, but it is also highly desirable that such new surfactants are environmentally-friendly, are liquids and possess little or no color.
There is a need for surfactants which exhibit good equilibrium and dynamic surface tension properties, are low-foaming, are low viscosity liquids to facilitate handling, have low color and low odor characteristics and would be widely accepted in the waterborne coating, ink, adhesive, fountain solution and agricultural formulation industries. Moreover, since there is substantial interest in the development of environmentally-friendly surfactants, an essential attribute would be that these surfactants not only possess the aforementioned desired performance attributes but also are derived from naturally occurring compounds or their synthetic equivalents or possess favorable biodegradation and toxicity properties.
The importance of reducing equilibrium and dynamic surface tension in applications such as coatings, inks, adhesives, fountain solutions and agricultural formulations is well-appreciated in the art.
Low dynamic surface tension is of great importance in the application of waterborne coatings. In an article, Schwartz, J. xe2x80x9cThe Importance of Low Dynamic Surface Tension in Waterborne Coatingsxe2x80x9d, Journal of Coatings Technology, September 1992, there is a discussion of surface tension properties in waterborne coatings and a discussion of dynamic surface tension in such coatings. Equilibrium and dynamic surface tension were evaluated for several surface active agents. It is pointed out that low dynamic surface tension is an important factor in achieving superior film formation in waterborne coatings. Dynamic coating application methods require surfactants with low dynamic surface tensions in order to prevent defects such as retraction, craters, and foam.
Efficient application of agricultural products is also highly dependent on the dynamic surface tension properties of the formulation. In an article, Wirth, W.; Storp, S.; Jacobsen, W. xe2x80x9cMechanisms Controlling Leaf Retention of Agricultural Spray Solutionsxe2x80x9d; Pestic. Sci. 1991, 33, 411-420, the relationship between the dynamic surface tension of agricultural formulations and the ability of these formulations to be retained on a leaf was studied. These workers observed a good correlation between retention values and dynamic surface tension, with more effective retention of formulations exhibiting low dynamic surface tension.
Low dynamic surface tension is also important in high-speed printing as discussed in the article xe2x80x9cUsing Surfactants to Formulate VOC Compliant Waterbased Inksxe2x80x9d, Medina, S. W.; Sutovich, M. N. Am. Ink Maker 1994, 72 (2), 32-38. In this article, it is stated that equilibrium surface tensions (ESTs) are pertinent only to ink systems at rest. EST values, however, are not good indicators of performance in the dynamic, high speed printing environment under which the ink is used. Dynamic surface tension is a more appropriate property. This dynamic measurement is an indicator of the ability of the surfactant to migrate to a newly created ink/substrate interface to provide wetting during high speed printing.
U.S. Pat. No. 5,098,478 discloses water-based ink compositions comprising water, a pigment, a nonionic surfactant and a solubilizing agent for the nonionic surfactant. Dynamic surface tension in ink compositions for publication gravure printing must be reduced to a level of about 25 to 40 dynes/cm to assure that printability problems will not be encountered.
U.S. Pat. No. 5,562,762 discloses an aqueous jet ink of water, dissolved dyes and a tertiary amine having two polyethoxylate substituents and that low dynamic surface tension is important in ink jet printing.
A variety of esters of malic acid (2-hydroxy-butanedioic acid), also called malates, are known. Malic acid itself is used primarily as an additive in beverages, candy and food. The commercial form (DL-malic acid) is produced from maleic anhydride and is classified as GRAS (Generally Recognized As Safe) by the U.S. Food and Drug Administration. In addition, the naturally occurring form (L-malic acid) is found in many fruits at low concentrations. DE 3 011 645 A1 discloses the use of mixed esters of hydroxycarboxylic acids as dispersion aids for the aqueous suspension polymerization of vinyl chloride. U.S. Pat. No. 3,927,073 discloses esters of dicarboxylic acids with polyhydroxy tertiary mines as both a detergent and a fabric softening agent.
DE 19 621 681 Al discloses aqueous pearl luster concentrates comprising esters of polyvalent carboxylic acids and/or hydroxycarboxylic acids with fatty alcohols in conjunction with emulsifiers and polyols. The mono- or di-esters of C6 to C22 alcohols were used to impart a pearl luster to xe2x80x9csurface active agentsxe2x80x9d for use in hair shampoo and manual dishwashing detergents. Among the suggested acids is malic acid.
U.S. Pat. No. 5,695,679 discloses dishwashing detergent formulations comprising esters of mono- or polycarboxylic acids and mono- or polyhydric alcohols as xe2x80x9corganic silver coating agentsxe2x80x9d. Malic acid is listed among the carboxylic acids.
U.S. Pat. No. 2,925,352 discloses derivatives of malic acids as plasticisers for water-insoluble thermoplastic organic film-forming polymers. Example 4 shows a film-forming dope comprising 100 parts cellulose acetate, 12.5 parts of diisobutyl malate, 400 parts of dioxalane and 27 parts water.
U.S. Pat. No. 2,122,716 discloses hydroxycarboxylic acid esters of C10-C14 alcohols.
C. D. Vaughan and D. A. Rice, J. Dispersion Science and Technology, 1990, 11, 83, show the use of dioctylmalate in an oil-in-water emulsion used to test an equation for the xe2x80x9cRequired HLBxe2x80x9d value for the oil phase in order to obtain a stable emulsion.
Kyotani et al, Sekiyu Gakkaishi, 1988, 31, 382, studied the effect of OH groups on the flow behavior of mono- and di-ester lubricants made from 2-ethylhexanol. The increased viscosity of di(2-ethylhexyl)malate versus di(2-ethylhexyl)succinate illustrates that the latter is a better lubricant. In this case, all liquids were studied neat and not in aqueous media.
U.S. Pat. No. 4,005,189 discloses as a deodorant an ester of an aliphatic mono- or dihydroxycarboxylic acid or an aliphatic mono- or dihydroxy-dicarboxylic acid having 2 to 4 carbons with an aliphatic alcohol having 1 to 6 carbons. Diethyl malate, diisopropyl malate and dihexyl malate are shown.
EP 0 850 935 A2 discloses concentrated solutions of 1,3,5-triazine derivatives and certain esters of carboxylic acids as solvents. A preferred solvent is bis(2-ethyl-hexyl) malate.
U.S. Pat. No. 5,505,937 discloses a transfer resistant cosmetic composition containing as a low viscosity oil dioctyl malate among the many listed esters.
U.S. Pat. No. 5,702,693 discloses an aqueous liquid composition for removal of gypsum from the skin of a patient comprising a water-miscible organic solvent, an acid and an emollient. The examples show dioctyl malate as an emollient.
U.S. Pat. No. 5,597,576 discloses oil-based transparent gels, i.e., xe2x80x9clipogelsxe2x80x9d, containing malic acid diesters of C12-13 single-branch fatty alcohols.
This invention provides water-based compositions containing an organic or inorganic compound, particularly aqueous organic coating, ink, adhesive, fountain solution and agricultural compositions, having reduced equilibrium and dynamic surface tension by incorporation of an effective amount of a diester of malic acid of the following structure: 
where R1 and R2 are C3 to C6 alkyl groups, but preferably R1 and R2 are the same. It is desirable that an aqueous solution of the malate diester demonstrates a dynamic surface tension of less than 45 dynes/cm at a concentration of xe2x89xa65 wt % in water at 25xc2x0 C. and 6 bubble/second according to the maximum-bubble pressure method. The maximum-bubble-pressure method of measuring surface tension is described in Langmuir 1986, 2, 428-432, which is incorporated by reference.
By xe2x80x9cwater-basedxe2x80x9d, xe2x80x9caqueousxe2x80x9d or xe2x80x9caqueous mediumxe2x80x9d, we mean, for purposes of this invention, a solvent or liquid dispersing medium which comprises at least 90 wt %, preferably at least 95 wt %, water. Obviously, an all water medium is also included.
Also provided is a method for lowering the equilibrium and dynamic surface tension of such aqueous compositions by the incorporation of these malate diester compounds.
Also provided is a method for applying a coating of a water-based inorganic or organic compound-containing composition to a surface to partially or fully coat the surface with the water-based composition, the composition containing an effective amount of a malate diester compound of the above structure for reducing the dynamic surface tension of the water-based composition.
There are significant advantages associated with the use of these malate diesters in water-based organic coatings, inks, adhesives, fountain solutions and agricultural compositions and these advantages include:
water-borne coatings, inks, adhesives, fountain solutions and agricultural compositions which may be applied to a variety of substrates with excellent wetting of substrate surfaces including contaminated and low energy surfaces;
a reduction in coating or printing defects such as orange peel and flow/leveling deficiencies;
coating and ink compositions capable of high speed application;
low-foam surfactants capable of reducing dynamic surface tension;
low-foam surfactants which are low viscosity liquids at room temperature for facile handling;
low-foam surfactants which have low odor and color;
water-borne coatings and inks which have low volatile organic content, thus making these formulations environmentally favorable;
water-borne compositions using a surfactant derived from natural acids or synthetic equivalents, thus making such compositions environmentally favorable; and
low-foam surfactants which exhibit good biodegradation characteristics and thus, are environmentally favorable.
Because of their excellent surfactant properties and the ability to control foam, these materials are likely to find use in many applications in which reduction in dynamic and equilibrium surface tension and low foam are important. Applications in which low foam is important include various wet-processing textile operations, such as dyeing of fibers, fiber souring, and kier boiling, where low-foaming properties would be particularly advantageous; they may also have applicability in soaps, water-based perfumes, shampoos, and various detergents where their marked ability to lower surface tension while simultaneously producing substantially no foam would be highly desirable.
This invention relates to the use of compounds of the structure 
where R1 and R2 are independently a C3-C6 alkyl group, preferably R1=R2, for the reduction of equilibrium and dynamic surface tension in water-based compositions containing an organic compound, particularly coating, ink, fountain solution, adhesive and agricultural compositions containing organic compounds such as polymeric resins, herbicides, fungicides, insecticides or plant growth modifying agents. It is desirable that an aqueous solution of the malate diester demonstrates a dynamic surface tension of less than 45 dynes/cm at a concentration of xe2x89xa65 wt % in water at 25xc2x0 C. and 6 bubble/second according to the maximum-bubble-pressure method. The maximum-bubble-pressure method of measuring surface tension is described in Langmuir 1986, 2, 428-432, which is incorporated by reference.
In one aspect of the invention the malate diesters of the above formula display excellent ability to reduce equilibrium and dynamic surface tension while producing substantially no foam.
These materials may be prepared by esterification of malic acid with an alcohol. The reaction is illustrated below: 
For the purpose of this invention all stereoisomers of malic acid are suitable, including L-malic acid, D-malic acid and DL-malic acid.
The esterification reaction may be performed using many catalysts and processes as described in the Kirk-Othmer Encyclopedia of Chemical Technology, 4th Ed., Vol. 9, p. 755-780, which is incorporated by reference. The reaction is preferentially catalyzed by an acid. Examples of suitable acid catalysts are acidic ion exchange resins (i.e. Amberlyst(copyright) 15 resin), p-toluenesulfonic acid, boron trifluoride etherate and mineral acid catalysts, such as hydrochloric acid and sulfuric acid. In addition, the esterification reaction may be driven by removal of the water by-product. In this case, the water may be removed as an azeotrope, typically with the alcohol used in the reaction. Other suitable methods to remove water include the use of a drying agent. Also, solvents may be added to the reaction to aid the dissolution of malic acid or to facilitate azeotropic removal of water.
All alcohols or mixtures of alcohols containing the requisite C3 to C6 alkyl substituents may be utilized for the preparation of the dialkylmalates of this invention with alcohols containing a 3-5 carbons being preferred and those containing 4 carbons being especially preferred. Alkyl groups which are suitable should have sufficient carbon to confer surface activity (i.e. an ability to reduce the surface tension of water) to the material but not enough carbon to decrease the solubility to the extent that the ability of the material to reduce surface tension is insufficient for a particular application. In general, an increase in the carbon number increases the efficiency of the resulting dialkylmalate (i.e. less surfactant is required to obtain a given decrease in surface tension), but decreases its ability to reduce surface tension at high surface creation rates. The latter effect is a result of the fact that increased carbon number generally decreases the water solubility of the material, and consequently, diminishes the diffusive flux of surfactant to newly-created surface. Generally, in the practice of this invention, it is desirable to choose alkyl groups such that the resulting dialkylmalates have a solubility limit in water from 0.005 to 5 wt %, preferably from 0.01 to 3 wt %, and most preferably from 0.1 to 1.0 wt %.
The alkyl groups in the malates of this invention may be the same or different. However, symmetrical malates are preferred due to ease in synthesis. Alkyl groups may be linear or branched, with alkyl groups containing terminal branching being preferred. The point of attachment to the oxygen may be on either an internal or terminal carbon, with a terminal carbon being preferred. The total number of carbons on R1 and R2 should be xe2x89xa66; fewer than this diminishes the surface activity of the dialkylmalate too greatly. The total number of carbons should be xe2x89xa612; a greater number decreases the solubility of the material to such a degree that its use in many formulations is impractical. Examples of suitable alkyl groups are n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, n-pentyl, 2-pentyl, 3-pentyl, isopentyl, neopentyl, cyclopentyl, 2-methylbutyl, 3-methyl-2-butyl, n-hexyl, 2-hexyl, 3-hexyl, cyclohexyl, 2-ethylbutyl, 4-methyl-2-pentyl and so on. Preferred derivatives are those in which R1=R2 and contain a total of 6 to 12 alkyl carbons. Of these derivatives those which contain 6 to 10 alkyl carbons are preferred and those containing 8 to 10 alkyl carbons especially preferred, with those containing 8 alkyl carbons being the most preferred, especially in the case where R1=R2=isobutyl. Moreover, those alkyl groups which contain terminal branching or attachment at terminal oxygen are preferred.
An amount of dialkylmalate compound that is effective to reduce the equilibrium and/or dynamic surface tension of the water-based, organic compound-containing composition is added. Such effective amount may range from 0.001 to 20 wt %, preferably 0.01 to 10 wt %, and most preferably 0.05 to 5 wt %, of the aqueous composition. Naturally, the most effective amount will depend on the particular application and the solubility of the dialkylmalate.
The malate diesters are suitable for use in an aqueous composition comprising in water an inorganic compound which is a mineral ore or a pigment or an organic compound which is a pigment, a polymerizable monomer, such as addition, condensation and vinyl monomers, an oligomeric resin, a polymeric resin, a detergent, a caustic cleaning agent, a herbicide, a fungicide, an insecticide, or a plant growth modifying agent.
In the following water-based organic coating, ink, adhesive, fountain solution and agricultural compositions containing a dialkylmalate according to the invention, the other listed components of such compositions are those materials well known to the workers in the relevant art.
A typical water-based protective or decorative organic coating composition to which the malate diester surfactants of the invention may be added would comprise in an aqueous medium 30 to 80 wt % of a coating composition containing the following components:
A typical water-based ink composition to which the malate diester surfactants of the invention may be added would comprise in an aqueous medium 20 to 60 wt % of an ink composition containing the following components:
A typical water-based agricultural composition to which the malate diester surfactants of the invention may be added would comprise in an aqueous medium 0.1 to 80 wt % of an agricultural composition containing the following components:
A typical water-based fountain solution composition would the following components:
A typical water-based adhesive composition to which the dialkylmalate surfactants of the invention may be added would comprise in an aqueous medium 30 to 65 wt % of an adhesive composition containing the following components:
With the exception of diisopropyl-(S)-(xe2x88x92)-malate and dibutyl-DL-malate, which were available commercially, all malates in the following examples were synthesized and characterized via Gas Chromatography/Mass Spectrometry (GC/MS) and Nuclear Magnetic Resonance (NMR) spectroscopy. All dialkylmalates prepared ranged from  greater than 96% to  greater than 99% pure.